Metal particulates have been conventionally utilized as circuit formation materials such as electronic material-oriented electroconductive pastes, for providing: a printed wiring; an internal wiring of semiconductor; a connection between a printed circuit board and an electronic component; and the like. Particularly, unlike typical particles larger than submicrons, metal particulates having diameters of 100 nm or less can be lowered in sintering temperature, thereby bringing about an idea to utilize metal particulates as low-temperature sintering pastes and the like.
Attention has been particularly and recently directed to techniques such that investigations and developments thereof have been progressed, the techniques each configured to draw a fine circuit pattern in paste or ink containing therein metal particulates by screen printing or by using an inkjet printer, for example, followed by low-temperature sintering to form circuits. Although developments adopting silver particulates have preceded the other developments in the present state, silver has an essential defect of migration upon usage thereof as a circuit formation material, thereby bringing about a demand for development of a copper particulate dispersion for a paste, ink, and the like.
Here, unlike other noble metals, copper has a nature susceptible to oxidation, so that conduction of technical development of a copper particulate dispersion requires consideration of oxidation resistance of copper particulate not only in a synthesis stage of particulates but also in a situation where the dispersion is prepared and used as an ink.
As such, there has been disclosed an electroconductive ink in Patent Document 1 based on a combination of metal colloidal particles and various additives, for example. Although such an electroconductive ink is allowed to utilize a mixed colloid containing, as a main component, silver therein which requires no consideration of oxidation resistance, the electroconductive ink is not allowed to utilize a mixed colloid containing, as a main component, copper which requires oxidation resistance, thereby still stopping short of solving the problem of oxidation resistance suffered by a copper particulate dispersion, in the present state.
Further, in case of drawing a circuit pattern by an inkjet printer, metal particulates contained in an ink are required to keep long-term dispersibility in the ink. As such, for metal particulates to be used in inks for inkjet printers, it has been typically conducted to coat metal particulates with a polymer dispersant, to guarantee long-term dispersion stability.
In general, as a method for producing fine particles which is suitable for mass-production, there is a method to synthesize fine particles using chemical reaction in a liquid phase. The method to synthesize fine particles is no exception for producing metal particulates as well. For example, a method for reducing metallic compounds in a solution by reducing agent such as hydrazine is well known.
Further, there is well known a polyol method (see Patent Document 2, for example), as a method for synthesizing metal particulates in a concentrated system having a higher productivity. The polyol method is configured to heat and reduce an oxide or complexes of copper within polyol, in a manner that the polyol plays three roles of a solvent, a reducing agent, and a protective agent, thereby enabling obtainment of metal particulates in submicron or micron orders even in the concentrated system.
To obtain metal particulates having diameters of 100 nm or less by means of such liquid phase based reduction, it is effective to coat surfaces of the particulates with water-soluble polymers such as polyvinyl pyrrolidone, polyvinyl alcohol, or gelatin so as to restrict growth of the particulates, and to avoid agglomeration of the particulates. To this end, many methods have been proposed, in such a manner to conduct addition of water-soluble polymers upon reductive reaction or after obtainment of particulates to thereby cause the polymers to be adsorbed onto the particulates (see Patent Document 3 and Patent Document 4, for example).
In turn, the present inventors have proposed methods each configured to obtain copper particulates by heating and reducing an oxide, hydroxide, or complexes of copper in a solution made of ethylene glycol, diethylene glycol, or triethylene glycol, in a manner to add noble metal ions for production of nuclei and to add water-soluble polymers as a dispersant such as polyvinyl pyrrolidone, polyethyleneimine, or the like, thereby obtaining copper particulates having diameters of 100 nm or less excellent in dispersibility (see Patent Document 5, Patent Document 6, and Patent Document 7, for example).
The metal particulate dispersions obtained according to the methods each contain metal particulates which have diameters of 100 nm or less coated with water-soluble polymers and which are stably dispersed in the dispersion, thereby also exhibiting an excellent oxidation resistance. As such, technical developments are being conducted to utilize the metal particulate dispersion in a state of ink or paste, in a manner to draw a fine circuit pattern by an inkjet printer or by means of screen printing and to heat and sinter the circuit pattern at 150 to 400° C. by utilizing a low-temperature sintering ability specific to metal particulates having diameters of 100 nm or less, thereby forming a circuit.
However, insofar as based on the dispersion containing therein metal particulates coated with water-soluble polymers, it has been difficult to achieve simultaneous establishment of a film-forming ability of the dispersion and an electroconductive property of a sintered film. Namely, since the coated macromolecule layer for keeping dispersibility rather acts as a factor to obstruct sintering, it has been impossible to achieve a sintered film having a sufficient electroconductivity insofar as based on a thickness of typically formed polymer coating layer. When sintering temperatures are raised to solve this problem, there are caused rapid thermal decomposition and evaporation of the polymer coating layer together with a considerable shrinkage thereof, thereby failing to form a continuous circuit.
In turn, excessively decreased thicknesses of polymer coating layers fail to keep dispersibility of metal particulates in a dispersion, and deteriorate oxidation resistance of metal particulates. As such, it becomes important to control an atmosphere upon sintering, particularly in case of copper particulates apt to be oxidized in the atmospheric air, such that introducing hydrogen or sintering in a vacuum atmosphere enables formation of an electroconductive film capable of withstanding practical use (see Patent Document 8, for example).
However, sintering in a hydrogen atmosphere or vacuum atmosphere leads to a complicated configuration of apparatus, a higher cost, and a problem of safety.
Under such circumstances, it is desired to provide a copper particulate dispersion for circuit formation, which adopts copper particulates excellent in migration resistance, which possesses an excellent dispersibility and oxidation resistance of copper particulates, which possesses an excellent film-forming ability such that sintering can be conducted under a simple condition of weakly oxidative atmosphere or the like, and which is also capable of achieving a satisfactory electroconductive property of a sintered film.    Patent Document 1: JP-A-2004-143325    Patent Document 2: JP-A-59-173206    Patent Document 3: JP-A-2004-232012    Patent Document 4: JP-A-2004-256857    Patent Document 5: JP-A-2005-097677    Patent Document 6: JP-A-2005-307335    Patent Document 7: JP-A-2005-330552    Patent Document 8: JP-A-2000-123634